Re: [PATCH] mm: avoid blocking lock_page() in kcompactd

[Matthew Wilcox]

On Mon, Jan 27, 2020 at 04:00:24PM +0100, Michal Hocko wrote:
> On Sun 26-01-20 15:39:35, Matthew Wilcox wrote:
> > On Sun, Jan 26, 2020 at 11:53:55AM -0800, Cong Wang wrote:
> > > I suspect the process gets stuck in the retry loop in try_charge(), as
> > > the _shortest_ stacktrace of the perf samples indicated:
> > > 
> > > cycles:ppp:
> > >         ffffffffa72963db mem_cgroup_iter
> > >         ffffffffa72980ca mem_cgroup_oom_unlock
> > >         ffffffffa7298c15 try_charge
> > >         ffffffffa729a886 mem_cgroup_try_charge
> > >         ffffffffa720ec03 __add_to_page_cache_locked
> > >         ffffffffa720ee3a add_to_page_cache_lru
> > >         ffffffffa7312ddb iomap_readpages_actor
> > >         ffffffffa73133f7 iomap_apply
> > >         ffffffffa73135da iomap_readpages
> > >         ffffffffa722062e read_pages
> > >         ffffffffa7220b3f __do_page_cache_readahead
> > >         ffffffffa7210554 filemap_fault
> > >         ffffffffc039e41f __xfs_filemap_fault
> > >         ffffffffa724f5e7 __do_fault
> > >         ffffffffa724c5f2 __handle_mm_fault
> > >         ffffffffa724cbc6 handle_mm_fault
> > >         ffffffffa70a313e __do_page_fault
> > >         ffffffffa7a00dfe page_fault
> > > 
> > > But I don't see how it could be, the only possible case is when
> > > mem_cgroup_oom() returns OOM_SUCCESS. However I can't
> > > find any clue in dmesg pointing to OOM. These processes in the
> > > same memcg are either running or sleeping (that is not exiting or
> > > coredump'ing), I don't see how and why they could be selected as
> > > a victim of OOM killer. I don't see any signal pending either from
> > > their /proc/X/status.
> > 
> > I think this is a situation where we might end up with a genuine deadlock
> > if we're not trylocking the pages.  readahead allocates a batch of
> > locked pages and adds them to the pagecache.  If it has allocated,
> > say, 5 pages, successfully inserted the first three into i_pages, then
> > needs to allocate memory to insert the fourth one into i_pages, and
> > the process then attempts to migrate the pages which are still locked,
> > they will never come unlocked because they haven't yet been submitted
> > to the filesystem for reading.
> 
> Just to make sure I understand. Do you mean this?
> lock_page(A)
> alloc_pages
>   try_to_compact_pages
>     compact_zone_order
>       compact_zone(MIGRATE_SYNC_LIGHT)
>         migrate_pages
> 	  unmap_and_move
> 	    __unmap_and_move
> 	      lock_page(A)

Yes.  There's a little more to it than that, eg slab is involved, but
you have it in a nutshell.